This invention relates to a method and apparatus for determining the position of satellites in or near geosynchronous orbits and more particularly, to improving GPS satellite coverage at geosynchronous altitude.
At the present time, many space missions use, or attempt to use, the Global Positioning System (GPS) for accurately determining the position of geosynchronous satellites or other satellites at or near geosynchronous orbits. For low orbit satellites, instant position determination can be achieved because a relatively large number of GPS satellites (up to a maximum of 12, and always more than 4) are visible at any time. Since the orbit of these satellites is below the orbits of the GPS satellites and the GPS constellation is designed for near-earth use, the coverage is excellent. The major problem that arises for satellites in a geosynchronous orbit (which is above the orbits of the GPS satellites) is that one (or less) GPS satellites are normally visible and never more than a maximum number of three GPS satellites are visible. For a more thorough discussion of the problem, refer to an article by C. C. Chao and H. Bernstein entitled xe2x80x9cOnboard Stationkeeping of Geosynchronous Satellites Using a Global Positioning System Receiverxe2x80x9d, Journal of Guidance, Control, and Dynamics, Vol. 17, No. 4, August 1994. Thus, at present the use of GPS for positioning of satellites in or near a geosynchronous orbit is tenuous at best.
Accordingly it is highly desirable to provide a method and apparatus for determining the position of satellites at or near geosynchronous orbits using GPS satellites.